Week 3b Chapter 12 X-ray Interaction with Matter 61

Week 3b Chapter 12 X-ray Interaction with Matter 61 -...

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Chapter 12 X-ray Interaction with Matter Electromagnetic Radiation interacts with structures with similar size to the wavelength of the radiation. Interactions have wavelike and particle like properties. X-rays have a very small wavelength, no larger than 10 -8 to 10 -9 .
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X-ray Interaction with Matter The higher the energy of the x-ray, the shorter the wavelength. Low energy x-rays interact with whole atoms. Moderate energy x-rays interact with electrons. High energy x-rays interact with the nuclei.
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Five forms of x-ray Interactions Classical or Coherent Scattering Compton Effect Photoelectric Effect Pair production Photodisintegration
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Two Forms of X-ray Interactions Important to Diagnostic X-ray Compton Effect Photoelectric Effect
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Classical or Coherent Scattering Low energy x-rays of about 10 keV interact in this manner. Incident photon interacts with the atom. There is a change in direction.
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Classical or Thompson Scattering There is no loss of energy and no ionization. Photon scattered forward. Because these are low energy x-rays, they are of little importance.
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Classical Scattering At 70 kVp only a few percent of the x-rays undergo this form of scattering. Classic Scatter may contribute to the graying of the image called film fog.
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Compton Effect Moderate energy x- ray photon through out the diagnostic x- ray range can interact with outer shell electron . This interaction not only changes the direction but
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Compton Effect reduced its energy and ionizes the atom as well. The outer shell electron is ejected. This is called Compton Effect or Compton Scattering.
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Compton Scattering The x-ray continues in an altered direction with decreased energy. The energy of the Compton-scattered x- ray is equal to the difference between the energy of the incident x-ray and the energy imparted to the electron.
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Compton Scattering The energy imparted to the electron is equal to its binding energy plus the kinetic with which it leaves the atom. During Compton-scattering most of the energy is divided between the scattered photon and the secondary electron. The Secondary Electron is called a Compton Electron.
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Compton Scattering The scattered photon and secondary electron will retain most of its energy so it can interact many times before it losing all of it’s energy.
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Compton Effect The scattered photon will ultimately be absorbed photoelectrically. The secondary electron will drop into a hole in the outer shell of an atom created by an ionizing event. Compton-scattered photons can be deflected in any direction .
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A zero angle deflection will result in no energy loss. As the angle approaches 180 degrees,
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Week 3b Chapter 12 X-ray Interaction with Matter 61 -...

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